The present invention relates to an improvement in the construction of a through-type capacitor, or more in particular to a reliable through-type capacitor most suited as a capacitor used for a high-frequency filter of a magnetron.
A construction of a through-type capacitor used with a filter of the magnetron is disclosed, for example, in No. JP-U-57-104446 filed for utility model registration in Japan by Tokyo Shibaura Electric Co., Ltd. on Dec. 18, 1980. The magnetron oscillates with high frequency, and therefore part of the fundamental oscillation electric wave or its high harmonics is liable to leak outside through power lines. To prevent this, a high-frequency cut-off filter is normally inserted in the connecting line between the magnetron and an external power supply. The filter is generally configured of a choke coil and a through-type capacitor with a filter case as an earth (ground) electrode. In the case of the magnetron used with the microwave oven, the through-type capacitor is repeatedly subjected to thermal stress by the heat generated in the microwave oven. Further, in an environment comparatively low in temperature, a wide range of heat fluctuations from low to high temperatures may adversely affect the service life of the through-type capacitor.
The problem in particular is that the different coefficients of thermal expansion among the members making up the through-type capacitor generates a mechanical stress due to thermal fluctuations in the through-type capacitor. If this stress would generate repeatedly over a long time, the internal structure of the capacitor would be broken.
Generally, in a through-type capacitor used as a dielectric member of ceramic material, the dielectric member is formed in cylindrical form, and a through hole for passing a central conductor (power line) is formed at the center of the cylinder. The inner peripheral surface of the through hole and the outer peripheral surface of the dielectric member are respectively galvanized with nickel to provide electrode surfaces. The central conductor is passed through the through hole and the central conductor is soldered with the electrode surface of the inner peripheral surface of the through hole. Further, the electrode on the outer peripheral surface of the dielectric member is soldered with the earth electrode. As a result, the entire clearance surrounded by the inner peripheral surface of the through hole and the central conductor is filled with solder. The coefficient of thermal expansion of strontium titanate (SrTiO.sub.3) or barium titanate (BaTiO.sub.3) used as material of the dielectric member is 6.2.times.10.sup.-6 /K (K: Absolute temperature), that of iron used as a material of the central conductor 11.76.times. 10.sup.-6 /K, that of copper 20.times.10.sup.-6 /K, and that of solder 21.times.10.sup.-6 /K. As seen from this, the coefficient of thermal expansion of the central conductor is different from that of the dielectric member to such an extent that the central conductor is thermally deformed to a degree different from the dielectric member. This difference in thermal deformation generates a compression stress with the heating of the capacitor, and a tension stress occurs at the time of cooling thereof along the diameter of the through hole of the dielectric member. When the capacitor is alternately heated and cooled in this way, stress would be repeatedly applied to the internal parts of the capacitor, thereby finally breaking the parts of the dielectric member.